1. Field of the Invention
The present invention relates to a shadow mask type color cathode ray tube (CRT) including an in-line type electron gun that is provided with a control grid of a quatrupole structure.
2. Description of the Prior Art
Beam spots are produced on a phosphor screen of a color CRT by impinging an electron beam thereon. Since the beam spots form picture elements of the regeneration image, they should be diametrically small and as round in shape as possible. Otherwise, satisfactory resolution could not be attained. However, the deflection yoke incorporated in a color CRT with an in-line type electron gun is constructed so as to be capable of self-convergence, so that the electron beam is subjected to a horizontal deflection magnetic field in a pin-cushion shape and a vertical deflection magnetic field in a barrel shape. This causes the beams spots, particularly those in the periphery of the phosphor screen, to be distorted diametrically.
To overcome this problem, at least one of a control grid, an acceleration grid, and a focusing grid is modified so as to shape its electron-beam through-hole to be non-circular, thereby forming an astigmatic lens field. One example is a quadrupole type control grid, such as one which is disclosed in Japanese Utility Model Publication No. 62-44448. The resulting astigmatic lens field enables a passing electron beam through a deflection magnetic field to take a horizontally elongated rectangular shape in cross-section, and minimizes a virtual image of the cross-over section of the electron beam.
A disadvantage of the quadrupole type control grid is that if the vertical diameters of the beam spots become diametrically small, the scanning beam is likely to interfere with the apertures of the shadow mask, thereby producing fatal moire effects in the regeneration image. Such moire effects are most conspicuous in a low electric current zone such as in a range of a few .mu. A to a few tens .mu. A.